Ribbons of water flow off a mock-up waterfall designed for the National September 11 Memorial & Museum, Friday, Jan. 22, 2010, at the Brooklyn Navy Yard in New York. The 30-foot waterfall will cascade down the sides of the footprints of the destroyed World Trade Center towers, forming reflecting pools surrounded by the terrorist attack victims' names. AP Photo/Mark Lennihan.

By: Ula Ilnytzky, Associated Press Writer

NEW YORK, NY (AP).- The waterfalls meant to evoke memories of the Sept. 11 dead stand three stories high in a field in Brooklyn, spilling down into a pool. Four large American flags poke through the grass to mark off each corner of a World Trade Center tower footprint.

For more than three weeks, builders of the Sept. 11 memorial have been tinkering with details like water flow and year-round heating with a mock-up of the cascades, part of Michael Arad's "Reflecting Absence" memorial at ground zero.

The architect saw the waterfalls  which will empty into huge reflecting pools set above the spots where the towers once stood  tested for the first time Friday as they were demonstrated for The Associated Press.

"One of the things I wanted the water and the design to do is to mark this continuous sense of absence," Arad said. "These voids, even though water falls into them ... they never fill up, they always remain empty, and that was very important to me," he added.

The waterfall-filled pools are the centerpiece of a memorial plaza that will take up half of the 16-acre site at ground zero. The pools will be surrounded by hundreds of sweetgum and white oak trees on a cobblestoned plaza; a memorial museum is being built below ground that will open about a year after the memorial.

The foundation overseeing the memorial raised $350 million privately to build it; the government agency that owns ground zero is spending hundreds of millions more on infrastructure at the site. Two of five planned skyscrapers and a multibillion-dollar transit hub are also under construction.

The waterfall mock-up being tested in Brooklyn is 40 feet wide, representing a corner of two walls of one pool  a fraction of its actual 176-foot perimeter. The waterfalls will drop 30 feet and then another 16 feet into a center void. Each will pump 26,000 gallons of water per minute.

Visitors on Friday looked up at the waterfalls. Once the memorials open at ground zero in lower Manhattan by the 10th anniversary of the 2001 attack, visitors will look down on them from street level. A low wall covered by bronze panels inscribed with nearly 3,000 victims' names will surround the pools.

Both the waterfalls and the name panels will be backlit at night, giving them a glow that will reinforce "the memorial is about the absence of the 2,982 people that were lost," said Joe Daniels, who heads the foundation overseeing the memorial.

The mock-up was created primarily to test the weir, a small dam with fingerlike structures that helps create a rope-like water flow. It's where the water falls from the plaza level down to the base of the pool.

"The testing was required so that just the right veil of water ... is exactly what the designers are hoping to achieve," Daniels said.

The weir, made of darkened stainless steel, will be installed at ground zero within a few months after the rest of the mock-up is dismantled next week.

Memorial builders have tested the waterfalls' mechanism in Brooklyn dozens of times, adjusting and tweaking the flow of the water and how it hit the pool. They even tested to see whether flowers and other mementos left by visitors would disrupt the flow if they got stuck between the grooves.

"We've got it to the point where we've conquered the beast," project manager Ron Vega said Friday.

Once the waterfalls are turned on, "our intention is to run it forever," he said.

Memorial builders will probably fully install the waterfalls by spring 2011 so that they run by Sept. 11, 2011, Vega said.

The Brooklyn site also has a mock-up of the wall that will support the name panels. Most of it is in wood, but a 10-foot section is fashioned with the materials that will be used at the memorial site: granite for the walls, bronze for the name panels.

Fictitious names were cut into the metal and left outdoors since March to determine how the bronze would react to outdoor elements, whether snow or leaves would get stuck in lettering.

"Something like 1,400 linear feet of bronze, a massive amount" will be used for the panels, said Daniels. Each pool will have 76 name panels.

The mock-up also resulted in cutting off the sharp edges of the wall corner, enabling wheelchairs to pull up close. After the change, Arad realized the twin towers had similar edges.

A smaller mock-up of the waterfalls was tested in Canada four years ago. Made of plywood, it was built to see how it would function in extreme cold and to document levels of the falling water.

Builders later decided to install heaters that could be turned on when the temperature drops below freezing. But hiding the device while still making it accessible presented a challenge, he said.

A design adjustment that slightly separated the falls at the uppermost corner took care of that, allowing the device to be hidden in that spot under the metal of the weir. The small alteration also made the two corner falls appear more distinct from one another.

"I'm hoping the sound of the water will create a sense of place and being somewhat buffered from the city beyond while still being firmly within it," Arad said, creating "an environment that is conducive to contemplation in the middle of the city."